1. Field of the Invention
The present invention relates to a knock control apparatus for an internal combustion engine, which apparatus is designed for sensing or detecting occurrence of knocking or knock event in the engine on the basis of level change of an ion current which flows by way of a spark plug upon combustion of an air-fuel mixture within the engine cylinder to thereby correct an engine control quantity for suppressing the knocking. More particularly, the invention is concerned with a knock control apparatus for an internal combustion engine, which apparatus is arranged for suppressing erroneous detection of knock event in a sooting state of the spark plug to thereby evade erroneous knock suppression control.
2. Description of Related Art
Heretofore, in the knock control apparatus for the internal combustion engine, the control quantity or quantities for the engine have been so controlled as to suppress knock occurrence (e.g. by retarding the ignition timing, a typical one of engine control quantities) upon detection of knock event with a view to protect the engine against damage or injury due to the knock occurrence.
Further, the knock control apparatus for the internal combustion engine in which the ion current flowing by way of the ignition plug is made use of for the detection of knock event is capable of detecting magnitude of the knock on a cylinder-by-cylinder basis without resorting to the use of a knock sensor, which is advantageous for realizing cost reduction. Heretofore, various types of knock control apparatuses operative on the basis of the ion current have been proposed.
In general, in the internal combustion engine, an air-fuel mixture charged into a combustion chamber defined within each of the engine cylinders is compressed by a piston moving reciprocatively within the cylinder. Subsequently, a high voltage is applied to a spark plug disposed within the cylinder and exposed to the combustion chamber, whereby a spark is generated between electrodes of the spark plug due to electric discharge. Thus, combustion of the compressed air-fuel mixture is triggered. Explosion energy resulting from the combustion is then converted into a movement of the piston in the direction reverse to that of the compression stroke, which motion is translated into a torque outputted from the engine via a crank shaft.
Upon combustion of the compressed air-fuel mixture within the combustion chamber in the engine cylinder, molecules prevailing within the combustion chamber are ionized. Thus, when a high voltage is applied to an ion current detecting electrode which is constituted by an electrode of the spark plug, migration of ions carrying electric charges takes place between the electrodes of the spark plug, which gives rise to generation of an ion current.
As is known in the art, magnitude of the ion current varies with a high sensitively in dependence on the variation in pressure within the combustion chamber and contains vibration components which are ascribable to the knock event. Thus, it is possible to decide on the basis of the ion current whether the knock event has occurred or not.
For having better understanding of the present invention, description will first be made of the technical background in some detail. FIG. 5 is a block diagram showing generally a configuration of a hitherto known or conventional knock control apparatus for an internal combustion engine which is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 9108/1998 (JP-A-10-9108). In the apparatus shown in FIG. 5, a high voltage is applied distributively to spark plugs of individual engine cylinders, respectively, through the medium of a distributor.
The conventional apparatus shown in FIG. 5 is so designed as to extract knocking vibration components superposed on an ion current i for counting knock pulses after waveform shaping of the known vibration components, to thereby make knock decision (i.e., decision as to knock occurrence) on the basis of the pulse counts number.
Referring to FIG. 5, there is provided in association with a crank shaft (not shown) of an internal combustion engine (not shown either and hereinafter also referred to simply as the engine) a crank angle sensor 1 which is adapted to output a crank angle signal SGT containing a number of pulses generated at a frequency which depends on a rotation number or speed (rpm) of the engine.
The leading edges of the individual pulses contained in the crank angle signal SGT indicate angular reference positions for the individual engine cylinders in terms of crank angles, respectively. The crank angle signal SGT is supplied to an electronic control unit (ECU) 2 which may be constituted by a microcomputer, to be used for performing various controls and arithmetic operations therefor.
The electronic control unit 2 includes a counter 21 for counting the number of pulses (also referred to as the pulses number) N of a knock pulse train Kp inputted from a waveform processing means (described later on) and a CPU (central processing unit) 22 for deciding presence or absence of knocking on the basis of the pulses number N.
The counter 21 and the CPU 22 cooperate with the waveform processing means to constitute a knock detecting means.
The electronic control unit 2 is so designed or programmed as to fetch the engine operation information signals from various sensors (not shown) as well as the crank angle signal SGT outputted from the crank angle sensor 1 and perform various arithmetic operations in dependence on the engine operation states, to thereby generate driving signals for a variety of actuators and devices inclusive of an ignition coil 4 and others.
An ignition signal P for the ignition coil 4 is applied to a base of a power transistor TR connected to a primary winding 4a of the ignition coil 4 for turning on/off the power transistor TR. More specifically, the power transistor TR is turned off in response to the driving signal P, whereby a primary current i1 is interrupted.
Upon interruption or breaking of the primary current i1, a primary voltage V1 appearing across the primary winding 4a rises up steeply, as a result of which a secondary voltage V2 further boosted up is induced in a secondary winding 4b of the ignition coil 4 and makes appearance thereacross as a voltage of high level for ignition which is usually on the order of several ten kilovolts. Hereinafter, this voltage will also be referred to as the high ignition voltage or simply as the ignition voltage.
In other words, the ignition coil 4 generates the secondary voltage V2 (high ignition voltage) in conformance with the ignition timings of the engine.
The distributor 7 which is connected to an output terminal of the secondary winding 4b operates so as to distribute and apply the secondary voltage V2 sequentially to spark plugs 8a, . . . , 8d mounted in the engine cylinders, respectively, in synchronism with the rotation of the engine, whereby spark discharges take place within combustion chambers defined within the engine cylinders, respectively, triggering combustion or burning of the air-fuel mixture confined within the combustion chamber.
More specifically, with the spark discharges occurring across the spark plugs 8a, . . . , 8d, respectively, in response to the application of the secondary voltage V2 in conformance with the ignition timing of the engine, the air-fuel mixture within the cylinders is fired or ignited.
Connected between the other end of the primary winding 4a of the ignition coil 4 and the ground is a series circuit which is composed of a rectifier diode D1, a current limiting resistor R, a capacitor 9 connected in parallel with a voltage limiting Zener diode DZ and a rectifier diode D2. The series circuit mentioned above constitutes a path for allowing a charging current to flow to the capacitor 9 which constitutes a bias voltage source serving for supplying a bias voltage for detecting an ion current.
More specifically, the capacitor 9 connected in parallel with the Zener diode DZ (i.e., connected between both terminals of the Zener diode) is electrically charged to a voltage level corresponding to a predetermined bias voltage VBi on the order of several hundred voltages by the charging current which flows under the primary voltage V1. Thus, the capacitor 9 serves as the bias voltage source for detecting the ion current i, as mentioned above. To this end, the capacitor 9 is so connected as to discharge by way of the spark plug (8a, . . . , 8d ) immediately after the ignition, allowing the ion current i to flow therethrough.
Connected to one end of the capacitor 9 are anodes of high-voltage diodes 11a, . . . ,11d, respectively, which have respective cathodes connected to one ends or electrodes of the spark plugs 8a, . . . , 8d, respectively, with a same polarity as that of the firing or ignition voltage. On the other hand, connected to the other end of the capacitor 9 is a resistor 12 for detecting the ion current, which serves to convert the ion current i into a voltage signal and output it as an ion current detection voltage signal Ei.
The resistor 12 is connected to the other ends of the spark plugs 8a, . . . , 8d, respectively, via the ground and forms a path for the ion current i in cooperation with the capacitor 9 and the high-voltage diodes 11a, . . . , 11d.
The ion current detection voltage signal Ei derived from the resistor 12 is shaped by a waveform shaper circuit 13 which thus outputs a waveform-shaped signal Fi. Subsequently, only the knock signal Ki is extracted from the waveform-shaped signal Fi through a band-pass filter 14. The knock signal Ki is then converted to a knock pulse train Kp through a comparison circuit 15 to be ultimately supplied to the counter 21 incorporated in the electronic control unit (ECU) 2.
The waveform shaper circuit 13, the band-pass filter 14 and the comparison circuit 15 cooperate to constitute a waveform processing means for extracting the knock pulse train Kp from the ion current detection voltage signal Ei.
The pulses number N of the knock pulse train Kp is counted by the counter 21 of the electronic control unit 2 to be used for allowing the CPU (central processing unit) 22 to make decision as to whether the knocking occurs or not.
The pulses number N of the knock pulse train Kp bears a significant correlation with the intensity or magnitude of knocking. In other words, the pulses number N increases, as the magnitude of knocking is larger.
Now, referring to FIG. 6 along with FIG. 5, operation of the conventional knock control apparatus for the internal combustion engine will be described.
FIG. 6 is a timing chart for illustrating waveforms of the various signals shown in FIG. 5. It can be seen from FIG. 6 that the knock signal is superposed on the ion current i (see waveform-shaped signal Fi).
The electronic control unit 2 outputs the ignition signal P for turning on/off the power transistor TR on the basis of the crank angle signal SGT derived from the output of the crank angle sensor 1. The power transistor TR electrically conducts (i.e., assumes ON-state) when the ignition signal P is at a high or "H" level, to thereby allow the primary current i1 to flow through the primary winding 4a of the ignition coil 4, while interrupting the current i1 at the time point when the ignition signal P changes from the "H" level to a low or "L" level.
Upon interruption of the primary current i1, the primary voltage V1 rising steeply is induced in the primary winding 4a, as a result of which the capacitor 9 is charged with the current flowing along the charging path constituted by the rectifier diode D1, the current limiting resistor R, the capacitor 9 and the rectifier diode D2. Needless to say, charging of the capacitor 9 is terminated when the voltage appearing across the capacitor 9 has reached the backward breakdown voltage (the bias voltage VBi) of the Zener diode DZ.
In this manner, the capacitor 9, the Zener diode DZ and the diode D2 cooperate to constitute a bias means, wherein the capacitor 9 is charged under the effect of the high voltage making appearance in the primary winding 4a (at low voltage side) upon interruption of the primary current i1.
When the primary voltage V1 makes appearance across the primary winding 4a, there is induced in the secondary winding 4b of the ignition coil 4 the secondary voltage V2 boosted up to a sufficiently high ignition voltage on the order of several ten kilovolts. This secondary voltage V2 is applied distributively to the spark plugs 8a, . . . , 8d of the individual engine cylinders, respectively, by way of the distributor 7, which results in generation of the spark discharge at the plugs within the combustion chambers of the engine cylinders which are under control. Thus, the air-fuel mixture undergoes combustion.
Upon combustion of the air-fuel mixture, ions are produced within the combustion chamber defined in the engine cylinder. The ion current i can thus flow under the bias voltage VBi charged in the capacitor 9. By way of example, let's assume that combustion of the air-fuel mixture takes place within the combustion chamber in which the spark plug 8a is disposed. Then, the ion current i flows along a current path extending from the capacitor 9 to the resistor 12 by way of the diode 11a and the spark plug 8a and then to the capacitor 9.
The ion current i is converted to the ion current detection voltage signal Ei by means of the resistor 12 (serving as the ion current detecting means), whereon the ion current detection voltage signal Ei is shaped to the waveform-shaped signal Fi through the shaper circuit 13.
As can be seen in FIG. 6, the shaped signal Fi is of such waveform which allows the knock signal Ki to be easily extracted by clipping only the ion current component at a predetermined voltage.
When a knocking event occurs in the engine, signal components ascribable to the knocking vibrations are superposed on the ion current i. Thus, the shaped signal Fi also assumes a waveform in which the knocking vibration components are superposed on the ion current.
The waveform-shaped signal Fi is supplied to the band-pass filter 14 and the comparison circuit 15 constituting the waveform processing means.
Thus, the band-pass filter 14 extracts or selects only the knock signal Ki indicating the knocking vibration frequency. On the other hand, the comparison circuit 15 compares the knock signal Ki with a predetermined reference level. As a result, a knock pulse train Kp is outputted from the comparison circuit 15 to be supplied to the counter 21 which is incorporated in the electronic control unit (ECU) 2.
The counter 21 of the electronic control unit 2 is designed to count the pulses number N of the knock pulse train Kp in response to a rising or falling edge of the knock pulse train Kp. The signal indicating the pulses number is then inputted to the CPU 22.
The pulses number N increases as the magnitude of the knocking becomes larger. Thus, the CPU 22 of the electronic control unit 2 can decide or determine the presence or absence of the knocking and magnitude thereof on the basis of the pulses number N.
By virtue of the feature described above, the control quantity (ignition timing) can be so corrected as to suppress the knocking on the basis of the pulses number N.
By way of example, when a count value of the pulses number N becomes equal to or greater than a predetermined number, it is decided that the knocking occurs. In that case, the ignition timing is correctively retarded by a predetermined quantity. Subsequently, when occurrence of the knocking is still decided in succession, the retard quantity is accumulatively increased progressively, which is stopped when the decision results in no occurrence of the knocking.
The predetermined number of pulses (pulses number) which is used as the reference value for comparison for the knock decision can be set to a value of e.g. 5 to 20, although it depends on the engine rotation number and the waveform-shaped level in the comparison circuit 15.
In this way, by determining the retard quantity for retarding correctively the ignition timing in dependence on the result of decision made by the CPU 22, the ignition timing for the cylinder in which the knocking has occurred can be corrected optimally, whereby occurrence of knocking can be suppressed effectively.
However, when the spark plug 8a; . . . ; 8d is in the sooting state in which an insulation resistance value of the plug gap through which the ion current i flows is lowered, a leak current determined by the insulation resistance value and the bias voltage can flow through the plug gap, as a result of which an ion current i containing the leak current will be detected.
In that case, the leak current may contain a lot of vibration components of high frequency which are apt to be detected erroneously as a knock signal. Consequently, there may arise such situation that optimal correction for the ignition timing can not be realized.
As will now be appreciated from the foregoing description, in the case of the knock control apparatus for the internal combustion engine known heretofore, the knock control based on the ion current i can certainly be carried out. However, because the counter measures for coping with occurrence of the sooting state of the spark plugs 8a, . . . , 8d is not provided, noise making appearance in the sooting state may be detected erroneously as the occurrence of knocking, giving rise to a problem.